Polymer electrolyte type fuel cell

ABSTRACT

A polymer electrolyte fuel cell of the present invention includes conductive separator plates comprising molded plates of a composition comprising a binder, conductive carbon particles whose average particle diameter is not less than 50 μm and not more than ⅓ of the thickness of the thinnest portion of the conductive separator plate, and at least one of conductive carbon fine particles and micro-diameter carbon fibers. The separator plates do not require conventional cutting processes for gas flow channels, etc., and can be easily mass produced by injection molding and achieve a reduction in the cost.

TECHNICAL FIELD

The present invention relates to a fuel cell using a polymer electrolytemembrane for use in portable power sources, electric vehicle powersources, domestic cogeneration systems, etc.

BACKGROUND ART

A fuel cell using a polymer electrolyte membrane generates electricpower and heat simultaneously by electrochemically reacting a fuel gascontaining hydrogen and an oxidant gas containing oxygen such as air.This fuel cell is basically composed of a polymer electrolyte membranefor selectively transporting hydrogen ions; and a pair of electrodes,i.e., an anode and a cathode, formed on both surfaces of the polymerelectrolyte membrane. The electrode usually comprises a catalyst layerwhich is composed mainly of a carbon powder carrying a platinum groupmetal catalyst and formed on the surface of the polymer electrolytemembrane; and a diffusion layer which has both gas permeability andelectronic conductivity and is formed on the outer surface of thecatalyst layer.

In order to prevent the fuel gas and oxidant gas supplied to theelectrodes from leaking out or prevent these two kinds of gases frommixing together, gaskets are arranged on the periphery of the electrodeswith the polymer electrolyte membrane therebetween. The gaskets arecombined integrally with the electrodes and the polymer electrolytemembrane beforehand. This is called “MEA” (electrolytemembrane-electrode assembly). Disposed outside the MEA are conductiveseparator plates for mechanically securing the MEA and for connectingadjacent MEAs electrically in series, or in some cases, in parallel. Theseparator plates have a gas flow channel for supplying a reaction gas tothe electrode surface and for removing a generated gas and an excessgas, in a portion that comes into contact with the MEA. Although the gasflow channel may be provided separately from the separator plates,grooves are usually formed on the surfaces of the separator plates toserve as the gas flow channel. Also, a method in which the gas flowchannel grooves are formed on the electrodes has been proposed,depending on the circumstances.

In order to supply the fuel gas and oxidant gas to these grooves, it isnecessary to use piping jigs which branch respective supply pipes forfuel gas and oxidant gas, according to the number of separator plates tobe used, and connect the branches directly to the grooves of theseparator plates. This jig is called “manifold”, and the above-describedtype, directly connecting the supply pipes for fuel gas and oxidant gaswith the grooves, is called “external manifold”. A manifold having asimpler structure is called “internal manifold”. In the internalmanifold, the separator plates with the gas flow channels formed thereonare provided with through holes which are connected to the inlet andoutlet of the gas flow channel such that the fuel gas and oxidant gasare supplied directly from these holes.

Since the fuel cell generates heat during operation, it needs coolingwith cooling water or the like to keep the cell under good temperatureconditions. Normally, a cooling section for flowing the cooling watertherein is formed every one to three cells. The cooling section isinserted between the separator plates in one structure, and the coolingsection is formed by providing the backsides of the separator plateswith a cooling water flow channel in the other structure. The latterstructure is often employed. In a general structure of a cell stack, theMEAs, separator plates and cooling sections are alternately stacked toform a stack of 10 to 200 cells, and the resultant stack is sandwichedby end plates with current collector plates and insulating plates and isclamped with clamping bolts from both sides.

In such a polymer electrolyte fuel cell, the separator plates need tohave high conductivity, high tightness against the fuel gas, and highcorrosion resistance against a reaction in hydrogen/oxygenoxidation-reduction. For such reasons, conventional separator plates aremade from a glassy carbon plate or a dense graphite plate, and producedby forming a gas flow channel on the surface thereof by cutting, or byplacing an expanded graphite powder together with a binder in a pressmold with a gas flow channel formed thereon and by heating/baking themafter pressing.

In recent years, there have been attempts to use a metallic plate suchas stainless steel in place of conventionally used carbon materials. Inthe case of the separator plate using a metallic plate, however, sincethe metallic plate is exposed to acidic atmosphere at high temperatures,corrosion and dissolution of the metallic plate will occur when used ina long time. If the metallic plate is corroded, the electricalresistance in the corroded portion increases, and the output of the celldecreases. Besides, if the metallic plate is dissolved, the dissolvedmetal ions diffuse into the polymer electrolyte and trapped at the ionexchange site of the polymer electrolyte, and consequently the ionicconductivity of the polymer electrolyte decreases. In order to preventsuch deteriorations, the surface of the metal plate is normally platedwith gold having a certain thickness. Furthermore, separator plates madeof a conductive resin obtained by mixing a metal powder with an epoxyresin or the like have been examined (for example, Japanese Laid-OpenUnexamined Patent Publication No. 6-333580).

As described above, in the conventional method of producing a separatorplate by cutting a glassy carbon plate or the like, the cost of thematerial of glassy carbon plate is high, and, further, it is difficultto reduce the cost of cutting the glassy carbon plate. In the case of aseparator plate produced by pressing expanded graphite, in order toretain the high conductivity of the separator plate, the content of theexpanded graphite in the separator plate needs to be made 80 wt % ormore. Accordingly, there arises a problem in the dynamic strength of thematerial. Therefore, the separator plate sometimes had cracks, whichwere caused by a deviation of the clamping load of the cell stack due toa variation in the thickness of the separator plate, more particularlyvibration and impact during driving when used as the power source of anelectric vehicle. If carbon fibers are added, the strength is improved,but it becomes difficult to perform injection molding as the flowabilityof a binder resin decreases. Furthermore, the metallic separator plateswith gold plating have a problem with the cost of the gold plating. Aseparator plate made from a conductive resin has a lower conductivitycompared to glassy carbon and metal plates, and the surface of the resinis hard. Therefore, in order to decrease the electric resistance in theportion in contact with the electrode, clamping needs to be performed ata higher pressure, and thus there is a problem that the cell structurebecomes complicated.

It is an object of the present invention to provide low-cost conductiveseparator plates having a low volume resistivity by improving conductiveseparator plates composed of a binder and a conductive materialconsisting mainly of conductive carbon particles.

The present invention also provides a method for manufacturing such aconductive separator plate.

DISCLOSURE OF INVENTION

The present invention provides a polymer electrolyte fuel cellcomprising conductive separator plates made of molded plates of acomposition comprising a binder, conductive carbon particles, and atleast one of conductive carbon fine particles and micro-diameter carbonfibers.

Here, the average particle diameter of the conductive carbon particlesis not less than 50 μm, and is not more than ⅓ of the thickness of thethinnest portion of the conductive separator plate, preferably not morethan 200 μm.

Preferred conductive carbon fine particles are carbon fine particleshaving a peak of the particle size distribution at an average diameterof 30 to 100 nm.

A preferred conductive micro-diameter carbon fiber is a carbon fiberhaving a diameter of 10 to 30 nm and a length of 1 to 10 μm.

The present invention provides conductive separator plates furthercomprising a metallic filler.

The present invention provides a method for manufacturing a conductiveseparator plate for use in a polymer electrolyte fuel cell, comprisingthe steps of preparing molding pellets comprising the above-mentionedcomposition, and injection molding the pellets.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a vertical cross sectional view showing an essential part ofan MEA used in a fuel cell of one example of the present invention.

FIG. 2 is a cathode-side front view of a separator plate used in thefuel cell of the same example.

FIG. 3 is an anode-side front view of the separator plate.

FIG. 4 is a cooling water-side front view of another separator plateused in the fuel cell of the same example.

FIG. 5 is an anode-side front view of the MEA used in the fuel cell ofthe same example.

FIG. 6 is a view showing a change in the output characteristics of afuel cell of Example 1 with time.

FIG. 7 is a view showing a change in the output voltage of a fuel cellof Example 3 with time.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention relates to a polymer electrolyte fuel cellcomprising: a hydrogen ion conductive polymer electrolyte membrane; apair of electrodes sandwiching the hydrogen ion conductive polymerelectrolyte membrane therebetween; and a pair of conductive separatorplates including means for supplying and discharging a fuel gas to andfrom one of the electrodes and supplying and discharging an oxidant gasto and from the other electrode, wherein the conductive separator platescomprise molded plates of a composition comprising a binder, conductivecarbon particles whose average particle diameter is not less than 50 μmand not more than ⅓ of the thickness of the thinnest portion of theconductive separator plate, and at least one of conductive carbon fineparticles and micro-diameter carbon fibers.

The conductive separator plates of the present invention have a lowelectric conductivity compared to glassy carbon plates and metal plates.However, since these conductive separator plates can be prepared byinjection molding, there is no need to perform the cutting processes forgas flow channels, etc., which were necessary in the production ofconventional separator plates, thereby achieving an improvement in theproductivity and a reduction in the cost.

By arranging the conductive carbon particles to be ⅓ or less than thethickness of the thinnest portion of the conductive separator plate, themoldability is improved, and the gas permeability of the resultantseparator plates decreases.

In the conductive separator plates of the present invention, theconductive carbon fine particles and/or micro-diameter carbon fibers aredispersed in the binder for bonding the conductive carbon particlestogether, thereby imparting conductivity to the binder.

A preferred conductive carbon particle has a length-to-width ratio(length/width), i.e., an aspect ratio, of not less than 2. Typicalpreferred carbon particles are those having an elongated shape likegrains of rice. A composition containing such carbon particles flowswell during the injection molding of the separator plates. Moreover, theparticles in the separator plates to be molded can be oriented atrandom, thereby improving the conductivity of the separator plates.

In a preferred embodiment of the present invention, the separator platesfurther comprise a metallic filler. The metallic filler performs thefunction of electrically connecting the carbon particles together.Consequently, the volume resistivity of the separator plates becomeslower.

Like the conductive carbon particles, a preferred metallic filler is notmore than ⅓ of the thickness of the thinnest portion of the conductiveseparator plate, more preferably not more than 200 μm. The preferredmetallic filler has a length-to-width ratio of not less than 2.

If the metallic filler that is exposed on the surface of separator plateis a material such as silver that is corroded in an acidic atmosphere,it is preferable to remove the filler by dissolving, etc.

In a preferred embodiment of the present invention, the binder is madeof a thermoplastic resin.

Examples of the thermoplastic resin are polyethylene, polystyrene,polypropylene, polymethyl methacrylate, polyethylene terephthalate,polycarbonate, polyamide, polyimide, polyvinyl alcohol, polyphenylenesulfide, polyether ketone, polyether imide, fluorocarbon resin, esterresin, liquid crystal polymer, aromatic polyester, polyacetal, andpolyphenylene ether.

In another preferred embodiment of the present invention, the binder ismade of a gastight elastic body.

The gastight elastic body preferably comprises a polymer elastic bodyincluding polyisobutylene represented by formula (1) or an ethylenepropylene random copolymer represented by formula (2) as a main-chainskeleton.

where X and Y are polymerizable functional groups, m is an integer notless than 1, representing the repetition number of isobutylene oligomer.

where X and Y are polymerizable functional groups, and 1 and m areintegers not less than 1.

As the conductive carbon particles, for example, natural graphite,artificial graphite, expanded graphite, and glassy carbon may be used.As the conductive carbon fine particles, carbon black such as acetyleneblack, ketjen black and mesophase carbon may be used.

Typical examples of the conductive micro-diameter carbon fibers arecarbon nano-tubes.

Examples of the metallic filler are silver, copper, aluminum, iron,nickel, lead, tin, titanium, zinc, gold, and alloys thereof.

A preferred composition of the separator plates of the present inventioncomprises 20 to 45 wt % of binder, 50 to 74 wt % of conductive carbonparticles, 0.5 to 10 wt % of conductive carbon fine particles and/orconductive micro-diameter carbon fibers.

In still another preferred embodiment, the composition further comprises0.5 to 15 wt % of metallic filler.

A method for manufacturing a conductive separator plate for use in apolymer electrolyte fuel cell of the present invention comprises thesteps of preparing molding pellets comprising the above-mentionedcomposition, and injection molding the pellets.

A molding die used here is preferably made from a material having a lowthermal conductivity and a high hardness. As the material of the moldingdie, carbon tool steel (SK material) is generally used from theviewpoint of the molding tact and strength. When the molding material isinjected into the die, the molten binder in the molding material israpidly cooled and hardens upon contact with the die with a temperaturenot higher than the melting point thereof. Since normal injectionmolding materials have a low thermal conductivity, rapid cooling isnecessary in order to increase the molding tact. Therefore, thetemperature for holding the die is determined by the die fillingperformance and the molding tact.

The composition for the molding of the separator plates of the presentinvention has a high thermal conductivity. Hence, when the compositionis injected into the die, its temperature rapidly decreases, the binderin the surface portion in contact with the die hardens, and the flow ofthe material is stopped. Consequently, the composition does not fillevery nook and corner of the die, and a molding defect occurs.Therefore, a material having a low thermal conductivity is used as thematerial of the die and the dissipation of heat from the injected moltenbinder is delayed, thereby delaying the hardening of the binder in theportion in contact with the die and ensuring filling of the die with themolding material.

The composition for the molding of the separator plates of the presentinvention comprises a large amount of conductive filler including carbonparticles so as to increase the conductivity. Therefore, the abrasion ofthe die increases. Accordingly, certain hardness is required.

Considering the above-mentioned facts, the present invention uses amaterial that satisfies both a low thermal conductivity and a highhardness. Materials having a thermal conductivity of not more than 26W/m/K and a surface hardness HRC of not less than 35 at 100° C. arepreferred. One of the preferred materials is stainless steel SUS630.Alternatively, it is possible to use a material obtained by coating thesurface of carbon tool steel with a ceramic having a high hardness and alow thermal conductivity, such as alumina and zirconia. In one exampleof the method of coating alumina, aluminum is deposited on the surfaceof the base material, i.e., the carbon tool steel, in advance andpartially diffused into the base material by heating at 500° C., andthen oxidized in the air. Thus, an alumina layer closely bonded to thebase material can be formed.

In the case where the material for the separator plate containsmicro-diameter carbon fibers, if the ends of the carbon fibers arearranged to project from the surface of the separator plate, theelectrical contact with the gas diffusion electrode is improved. In onemethod for manufacturing such a separator plate, the micro-diametercarbon fibers are deposited on the inner surface of the molding dietogether with a releasing agent, and the carbon fibers are transferredto the surface of the separator plate to be molded. In another method,the ends of the carbon fibers are exposed on the surface of theseparator plate by burning off the surface portion of the moldedseparator plate by heating.

Next, the following description will explain in further detail apreferred embodiment of the present invention using a gastight elasticbody as the binder.

A conductive gastight elastic body constituting a conductive separatorplate is produced, for example, by mixing, as conductive materials,carbon nano-tubes as well as conductive carbon particles into an elasticbody comprising a polymer represented by formula (1) or (2) as a basematerial. By adding a small amount of carbon nano-tubes as theconductive material, such a separator plate can have a sufficientconductivity even when the ratio of the conductive material in theseparator plate is reduced to 75 wt % or less. Accordingly, the rigidityof the separator plate is improved, and cracks in the separator platedue to vibration can be significantly reduced.

Since the surface of the separator plate comprising the conductivegastight elastic body has flexibility and elasticity, even if theclamping load of cell stack is decreased, it is possible tosignificantly reduce the contact resistance between the separator platesand the electrodes and between the separator plates. In some cellstructure, since the separator plates themselves have flexibility andelasticity, gaskets to be provided on the periphery of the electrodesare not particularly necessary, and the gas seal between the separatorplates and the MEA can be directly provided by the surfaces of theseparator plates. It is therefore possible to decrease the directmaterial cost and the manufacturing processes by a reduction in thenumber of component parts. Moreover, even when this separator plate isvibrated in the state where the cell stack pressure is being applied, itwill never have cracks like carbon plates. Furthermore, by selecting abase material and a conductive material for the conductive gastightelastic body, the separator plate will never have corrosion under anacidic atmosphere, which occurs on metal separator plates.

It is also possible to add a vulcanizing agent having no sulfurcomponents.

The present inventors looked for a base material that specificallysatisfies the above-mentioned requirements, and found that a polymerelastic body comprising polyisobutylene represented by the formula (1)or an ethylene propylene random copolymer represented by the formula (2)as a main-chain skeleton is particularly suitable for the material ofthe separator plate having excellent gastightness, acid resistance andheat resistance.

The polymer elastic body comprising the formula (1) or (2) as amain-chain skeleton can realize an optimum elasticity for the separatorplates of the polymer electrolyte fuel cell by selecting the degree ofpolymerization. A conductive material is mixed into the resin or polymerelastic body in a liquid state before polymerization, and the resultantmixture is molded into a sheet form and hardened by application of heator an electron beam. Moreover, it is possible to form grooves forsupplying fluids such as a fuel gas when molding the mixture into asheet form. In this aspect, the processing steps during the manufactureof the separator plates can be significantly reduced compared to theconventional carbon plates and metal plates.

The polymer represented by the formula (1) shown above is such one asmade in a manner that isobutylene oligomers each, as one unit, having arepetition number m and having terminal functional groups X and Y addedthereto are cross-linked at the terminal functional groups. When allylgroups, acryloyl groups, methacryloyl groups, isocyanate groups, orepoxy groups are used as X and Y, and these functional groups are madecrosslink points, post-polymerization polymers are cross-linked in amatrix form to have a network structure as these functional groups arepolyfunctional groups. The physical properties of the polymers arelargely affected by the repetition number m of isobutylene oligomer inthe stage of raw material, contained in the polymer material representedby the formula (1), the final polymerization degree, and the kinds ofthe terminal functional groups.

According to the results of examination by the present inventors, whenthis material is used for the material of the conductive separatorplates of the polymer electrolyte fuel cell, the repetition number m ofisobutylene oligomer in the stage of raw material is preferably between56 and 72, and 64 on average, while the final polymerization degree ispreferably 8000 or more. In addition, the ratio of the content of theterminal functional groups X and Y is preferably as small as possiblefrom the viewpoint of stability and acid resistance.

As the terminal functional groups X and Y in the ethylene propylenerandom copolymer represented by the formula (2) above, diene groups,triene groups, diolefine groups, polyalkenyl cycloalkane groups,norbornene derivatives, acryloyl groups, methacryloyl groups, isocyanategroups, epoxy groups, or the like are used, and the material can behardened by a suitable polymerization reaction. When the terminalfunctional groups are diene groups, acryloyl groups or methacryloylgroups, the material can be hardened by electron beam irradiation. Whenthe terminal functional groups are isocyanate groups, the material canbe hardened by urethane bonding with the aid of water. When the terminalfunctional groups are epoxy groups, the material can be hardened byheating using a known amine based hardener such as ethyl diamine. Thephysical properties of the polymer are affected by l and m in theformula (2), the overall polymerization degree l+m, and the terminalfunctional groups. It is preferred that l is not more than 1000, m isnot more than 19000, and l+m is between 5000 and 20000.

As the conductive material to be mixed into the polymer elastic bodythat is the base material comprising the polyisobutylene represented bythe formula (1) or the ethylene propylene random copolymer representedby the formula (2) as the main-chain skeleton, various kinds ofconductive carbon powders and fibers as well as carbon nano-tubes aresuitably used. These conductive materials preferably contain carbonparticles with an average particle diameter of 10 to 200 μm. Bycontaining large carbon particles with an average particle diameter of10 μm or more, the contact resistance between the carbon particles canbe reduced. Besides, large particles exceeding 200 μm are not preferredbecause the flowability of the carbon particles deteriorates duringmolding. 50 to 100 μm particles are most preferred. An appropriate ratioof the content of the conductive material in a conductive polymerelastic body obtained by mixing the conductive material is 55 to 75 wt%, and an appropriate ratio of the carbon nano-tubes in the conductivematerial is 2 to 50 wt %. When the ratio of the carbon nano-tubes isless than 2 wt %, the contact between the carbon nano-tubes is notsufficient, and therefore the effect of improving the conductivity issmall. Moreover, since the carbon nano-tubes are expensive, it isdisadvantages to use the carbon nano-tubes in an amount exceeding 50 wt%.

A preferred composition for molding the separator plates of the presentinvention comprises a binder, conductive carbon particles with adiameter of 50 to 200 μm, and carbon nano-tubes. When pellets orparticulate matter prepared from this composition are injection molded,the conductive carbon particles are stacked in layers with high density,and the carbon nano-tubes are present at random around the conductivecarbon particles. In general, the volume resistivity of the separatorplate tends to increase as the number of layers of the conductive carbonparticles increases. By increasing the average particle diameter of theconductive carbon particles, it is possible to reduce the number of thelayers. However, since the number of contact points between the carbonparticles is also reduced, a significant effect can not be expected inreducing the volume resistivity. In the present invention, since thecarbon nano-tube is present between the conductive carbon particles, thenumber of contact points between the carbon particles increases, and thevolume resistivity is significantly decreased. Besides, when thecomposition containing such large carbon particles and carbon nano-tubesin the form of short fibers is injection molded, the carbon particlesand the carbon nano-tubes collide with each other when injected, and thedirection of the major axis of the carbon nano-tubes tends to be random.Therefore, the anisotropy of the resistance which occurs due to theorientation of the carbon fibers can be solved, and excellent electricconductivity can be obtained in both the plane direction and thicknessdirection of the separator plates.

The following description will explain an embodiment of the presentinvention with reference to the drawings.

FIG. 1 is a vertical cross sectional view of the essential part, showingthe structure of an MEA. 11 is a gas diffusion layer made of carbonpaper, 12 is a catalyst layer formed on one surface of the gas diffusionlayer 11, and the combination of the gas diffusion layer 11 and thecatalyst layer 12 is called an electrode 13. By sandwiching a polymerelectrolyte membrane 14 between a pair of the electrodes, an MEA 15 isconstructed.

FIG. 2 is a front view of a conductive separator plate, seen from thecathode side, and FIG. 3 is a rear view thereof, i.e. a front view seenfrom the anode side. This conductive separator plate 20 serves as acathode-side conductive separator plate and an anode-side conductiveseparator plate. The conductive separator plate 20 has, on one endthereof, an inlet-side manifold aperture 23 a for an oxidant gas, aninlet-side manifold aperture 24 a for a fuel gas, and an inlet-sidemanifold aperture 25 a for cooling water, and has, on the other endthereof, an outlet-side manifold aperture 23 b for the oxidant gas, aninlet-side manifold aperture 24 b for the fuel gas, and an outlet-sidemanifold aperture 25 b for the cooling water. The separator plate 20 hasa groove 26 formed to run from the manifold aperture 23 a to 23 b on asurface thereof facing the cathode. Provided in the groove are a rib 27for parting the separator plate 20 in the middle, and a group of ribs 28for forming a plurality of parallel gas flow channels 29.

On the other hand, the separator plate 20 has a groove 30 formed to runfrom the manifold aperture 24 a to 24 b on a surface thereof facing theanode. Provided in the groove are a rib 31 for parting the separatorplate 20 in the middle, and a group of ribs 32 for forming a pluralityof parallel gas flow channels 33.

The conductive separator plate 20 illustrated here is to be insertedbetween unit cells, and the cathode-side separator plate positioned onone end of a cell stack, which is obtained by stacking a plurality ofunit cells, has gas flow channels as shown in FIG. 2 on one surfacethereof, but has a plane surface on the other surface. Besides, theanode-side separator plate positioned on the other end of the cell stackhas gas flow channels as shown in FIG. 3 on one surface thereof, but hasa plane surface on the other surface.

FIG. 4 is a front view of a surface of a conductive separator plate,having a cooling water flow channel. Like the separator plate 20, thisconductive separator plate 41 has, on one end thereof, an inlet-sidemanifold aperture 43 a for the oxidant gas, an inlet-side manifoldaperture 44 a for the fuel gas, and an inlet-side manifold aperture 45 afor the cooling water, and has, on the other end thereof, an outlet-sidemanifold aperture 43 b for the oxidant gas, an inlet-side manifoldaperture 44 b for the fuel gas, and an outlet-side manifold aperture 45b for the cooling water. The separator plate 41 has, on one surfacethereof, a groove 46 running from the manifold aperture 45 a to 45 b toform the cooling water flow channel, and a plurality of circular ribs 47provided in the groove 47.

A pair of the conductive separator plates 41 is joined together so thattheir surfaces having the cooling water flow channels 46 face eachother, so that a cooling section for passing the cooling water is formedbetween them. Moreover, like FIG. 2, an oxidant gas flow channel runningfrom the inlet-side manifold aperture 43 a to the manifold aperture 43 bis formed on the rear surface of one of the separator plates, while,like FIG. 3, a fuel gas flow channel running from the inlet-sidemanifold aperture 44 a to the manifold aperture 44 b is formed on therear surface of the other separator plate.

FIG. 5 is a front view of an MEA. The MEA 50 comprises a polymerelectrolyte membrane 51, and electrodes 52 sandwiching the polymerelectrolyte membrane 51 therebetween. The polymer electrolyte membrane51 has, on one end thereof, an inlet-side manifold aperture 53 a for theoxidant gas, an inlet-side manifold aperture 54 a for the fuel gas, andan inlet-side manifold aperture 55 a for the cooling water, and has, onthe other end thereof, an outlet-side manifold aperture 53 b for theoxidant gas, an outlet-side manifold aperture 54 b for the fuel gas, andan outlet-side manifold aperture 55 b for the cooling water.

In the examples illustrated below, 50 cells were stacked by stacking theMEAs 50 shown in FIG. 5 with the separator plate 20 therebetween andinserting a pair of separator plates 41 for forming the cooling sectionevery two cells.

The following description will explain some examples of the presentinvention with reference to the drawings.

EXAMPLE 1

First, an electrode catalyst was prepared by causing a carbon blackpowder to carry platinum particles with an average particle diameter of30 Å in a weight ratio of 50:50. A dispersion of a perfluorocarbonsulfonic acid represented by the formula (3) in an ethyl alcohol wasmixed with a dispersion of this catalyst powder in isopropanaol so as toform a catalyst paste.

where m=1, n=2, 5≦x≦13.5, y≈1000.

Meanwhile, a water repellent treatment was applied to a carbon paper tobe a supporting body for the electrode. After soaking a carbon nonwovenfabric with outer dimensions of 8 cm×10 cm and a thickness of 360 μm(TGP-H-120 manufactured by Toray Industries Inc.) in an aqueousdispersion of fluorocarbon resin (Neoflon ND1 manufactured by DAIKININDUSTRIES, LTD.), the carbon nonwoven fabric was dried and heated for30 minutes at 400° C. to impart water repellency. By applying thecatalyst paste to one surface of this carbon nonwoven fabric by a screenprinting method, a catalyst layer was formed. A part of the catalystlayer is buried in the carbon nonwoven fabric. In this manner, theelectrode composed of the catalyst layer and the carbon nonwoven fabricwas produced. Adjustments were made so that the amounts of platinum andperfluorocarbon sulfonic acid contained in the electrode were both 0.3mg/cm².

Next, a pair of the electrodes was joined to the front and rear surfacesof a hydrogen ion conductive polymer electrolyte membrane with outerdimensions of 10 cm×20 cm by hot pressing so that the catalyst layerscame into contact with the electrolyte membrane. This is called anelectrolyte membrane-electrolyte assembly (MEA). Here, as the hydrogenion conductive polymer electrolyte membrane, a 30 μm-thick thin film ofperfluorocarbon sulfonic acid represented by the formula (3)(where m=1,n=2, 5≦×≦13.5, y≈1000) was used.

Next, the following description will explain a conductive separatorplate comprising a conductive gastight elastic body having an acidresistance.

A liquid raw material represented by the formula (1), where therepetition number m of isobutylene oligomer is 56 to 72, 64 on average,and the functional groups X and Y are both isoprene, was prepared. 100 gof the liquid raw material was mixed with conductive materials of 15 gof carbon nano-tubes, 100 g of a graphite powder with an averageparticle diameter of 80 μm, 50 g of a graphite powder with an averageparticle diameter of 1 μm or less, and 50 g of fibrous graphite (with anaverage diameter of 50 μm and an average length of 0.5 mm), and 200 g ofmethyl ethyl ketone was added to the mixture for viscosity adjustment.The mixture was mixed sufficiently to prepare a formulated concentratefor the separator plate. This formulated concentrate was poured into adie made of stainless steel and kept at 50° C. under a reduced pressureof an atmospheric pressure of 0.2 for one hour so as to volatilize themethyl ethyl ketone. Next, an acceleration voltage of 500 keV and anelectron beam with an exposure of 50 Mrad were applied to the mixture soas to polymerize the isoprene in the ends of the isobutylene oligomer,thereby producing the conductive separator plate. The degree ofpolymerization was about 10000.

When the repetition number m of isobutylene oligomer of the raw materialwas made smaller than 56, the resultant sheet after polymerization washard, and it was therefore necessary to increase the clamping pressurein assembling the cell in order to lower the contact resistance with theMEA. On the other hand, when m was made larger than 72, the sheet becametoo soft and the grooves of gas flow channel on the surface of separatorplate were squashed by the clamping pressure during the assembly of thecell. The influence by the degree of polymerization was examined bycontrolling the exposure of the electron beam to the raw material. As aresult, when the degree of polymerization was smaller than 5000, theresultant sheet was too soft and the grooves of gas flow channel weresquashed like the above.

Besides, it was confirmed that one obtained by using allyl groups,acryloyl groups, methacryloyl groups, isocyanate groups, or epoxy groupsother than isoprene as the terminal functional groups and hardening thematerial by a suitable polymerization reaction can be used in the samemanner. Note that, when acryloyl groups or methacryloyl groups were usedas the terminal functional groups, the material was hardened by electronbeam irradiation like the above; when isocyanate groups were used, thematerial was hardened by urethane bonding with the aid of water; or whenepoxy groups were used, the material was hardened by heating using aknown amine-based hardener such as ethyl diamine. In these cases, likethe case where the functional groups are allyl groups, when therepetition number m of isobutylene oligomer in the raw material stage,contained in the structure represented by the formula (1), was 56 to 72and the final polymerization degree was 8000 or more, a suitablematerial for the separator plate was obtained.

In the manner described above, the conductive separator plate 20 and theconductive separator plates 41 forming the cooling section shown inFIGS. 2 through 4 were produced. During the production of the separatorplate, the die made of stainless steel was processed so as to enable theformation of the gas flow channel grooves and manifold apertures in theseparator plates, and therefore it is not necessary to performpost-processing, such as cutting and press punching, after removing theseparator plate from the die.

The conductive separator plate has a size of 10 cm×20 cm and a thicknessof 2 mm. The grooves 29 and 33 of the separator plate 20 have a width of2 mm and a depth of 0.7 mm, and each of the ribs 28 and 33 between thegrooves has a width of 1 mm. The depth of the groove 46 of the separatorplates 41 is 0.7 mm. Besides, this example employed a gas sealing methodin which the outer peripheral edge of the separator plate and theperipheral edges of the manifold apertures (except portions connectedwith the gas flow channels) were made higher than the electrode contactsurface, i.e., the top faces of the ribs 28 and 32, by 0.3 mm, and thepolymer electrolyte membrane was sandwiched between the heightenedportions. Therefore, in the cell of this example, a gasket was notprovided on the periphery of the electrode on the MEA side.

With the use of the above-described MEA and separator plates, a cellstack was assembled by stacking 50 cells. On both ends of the cellstack, current collector plates of stainless steel, insulating platesand end plates were stacked and fixed with clamping rods. The clampingpressure per area of the separator plate was 3 kgf/cm². Compared to afuel cell using conventional carbon separator plates that requires ahigh clamping pressure of about 20 kgf/cm², predeterminedcharacteristics were obtained with a small pressure according to thepresent invention. However, when a pressure smaller than this pressurewas used, gas leakage occurred, the contact resistance increased, andthe cell performance deteriorated. On the other hand, when the cellstack was clamped too tightly, the protruding parts of the separatorplates were squashed and the flow of gases and cooling water wasimpaired, and consequently the cell performance also deteriorated. Inother words, it was important to adjust the clamping pressure by theelasticity of the conductive separator plates.

The polymer electrolyte fuel cell of this example thus produced was heldat 80° C., and a hydrogen gas humidified and heated to a dew point of80° C. was supplied to the anode, while the air humidified and heated toa dew point of 70° C. was supplied to the cathode. As a result, an opencircuit voltage of 50 V was obtained during no load at which a currentis not output to the outside.

FIG. 16 shows the change in the output characteristics with time when acontinuous power generation test was performed under the condition thatthe fuel gas utilization ratio was 80%, the oxygen utilization ratio was50% and the current density was 0.5 A/cm². As a result, it was confirmedthat the cell of this example maintained a cell output of 1000 W (22V-45 A) over 8000 hours.

Additionally, since the cell of this example was constructed bysandwiching the electrode sheet between the separator plates havingelasticity, it was particularly strong against vibration and impact.When a cell composed of conventional carbon separator plates was droppedfrom a height of 2 m, the separator plates cracked after about one drop,while the cell of this example did not have unrecoverable damage, exceptthat the rod in the clamped section was loosened, even after performingthe drop test 100 times.

EXAMPLE 2

In the above-mentioned example, while the material includingpolyisobutylene as a main-chain skeleton was used as the conductivegastight elastic body for producing the separator plates, Example 2 useda material prepared by mixing a conductive material into a base materialthat is a polymer elastic body including an ethylene propylene randomcopolymer represented by the formula (2) as a main-chain skeleton.

A liquid oligomer including the ethylene propylene random copolymerrepresented by the formula (2), where the terminal groups X and Y werediene groups, the copolymerization ratio was l:m=1:1, and l+m was about60, was prepared. 20 g of carbon nano-tubes, 100 g of a graphite powderwith an average particle diameter of 70 μm, 50 g of a carbon blackpowder, and 30 g of fibrous graphite (with an average diameter of 50 μmand an average length of 0.5 mm) were sufficiently mixed into 100 g ofthe prepared liquid oligomer so as to produce a formulated concentratefor the separator plate. This formulated concentrate was injectionmolded in a die made of stainless steel kept at 160° C., and then heldfor 10 minutes for vulcanization, thereby producing the conductiveseparator plate. The degree of polymerization l+m was about 7000.

Note that, when the degree of polymerization was made larger than 20000,the resultant sheet was too hard, and it was therefore necessary toincrease the clamping pressure in assembling the cell in order to lowerthe contact resistance with the MEA. On the other hand, when the degreeof polymerization was made smaller than 4000, the sheet became too softand the grooves of gas flow channel on the surface of separator platewere squashed with the clamping pressure during the assembly of thecell. Furthermore, the influence of the degree of polymerization wasexamined by controlling the exposure of the electron beam, andconsequently it was found that, when the degree of polymerization wassmaller than 5000, the resultant sheet was too soft and the grooves ofgas flow channel were squashed like the above.

In addition, it was confirmed that materials which have triene groups,diolefine groups, polyalkenyl cycloalkane groups, nobornene derivatives,acryloyl groups, methacryloyl groups, isocyanate groups, or epoxy groupsas the terminal functional groups in place of the diene groups andhardened by a suitable polymerization reaction can be used in the samemanner. Note that, when the acryloyl groups or methacryloyl groups wereused as the terminal functional groups, the material was hardened byelectron beam irradiation like the above; when the isocyanate groupswere used, the material was hardened by urethane bonding with the aid ofwater; or when the epoxy groups were used, the material was hardened byheating using a known amine-based hardener such as ethyl diamine.

With the use of the above-mentioned separator plates, a cell similar toExample 1 was assembled, and the characteristics thereof were evaluatedunder the same conditions as in Example 1. As a result, it was confirmedthat the cell had the characteristics as good as those of the cell ofExample 1. Moreover, the cell had excellent vibration resistance andimpact resistance like the cell of Example 1.

EXAMPLE 3

As the electrode catalyst for forming the catalyst layer, one comprisingan acetylene black powder carrying platinum particles with an averageparticle diameter of about 30 Å in a weight ratio of 75:25 was used. Theelectrolyte membrane had a film thickness of 50 μm, and the amounts ofplatinum and perfluorocarbon sulfonic acid contained in the catalystlayer were 0.5 mg/cm² and 1.2 mg/cm², respectively. Except for theabove-mentioned conditions, the MEA was produced in the same manner asin Example 1.

Next, the conductive separator plates were produced as follows. Pelletsfor the separator plates were produced by sufficiently heating andkneading 20 g of polyphenylene sulfide as a binder, 75 g of conductivecarbon particles with an average particle diameter of 50 to 200 μm, and5 g of carbon nano-tubes with a fiber diameter of 10 nm to 30 nm and afiber length of 1 to 10 μm. The pellets were put into an injectionmolding machine, and injection molded in a predetermined die to producethe conductive separator plate. The injection pressure was 1600 kgf/cm²,the die temperature was 150° C., and the molding time was 20 seconds.

In the above-described method, the conductive separator plates 20 and 41shown in FIGS. 2 through 4 were produced. The conductive separatorplates have a size of 10 cm×20 cm and a thickness of 4 mm. The grooves29 and 33 of the separator plate 20 have a width of 2 mm and a depth of1.5 mm, and each of the ribs 28 and 32 between the grooves has a widthof 1 mm. The depth of the grooves 46 of the separator plates 41 is 1.5mm.

Table 1 shows the results of measuring the volume resistivity of theresultant separator plates. Compared to a separator plate produced froma composition containing no carbon nano-tubes, the volume resistivity ofthe separator plates of this example was reduced to 1/100 or less, andwas not higher than 20 mÙ·cm.

TABLE 1 No. 1-1 1-2 Composition Binder  20 20 (wt %) Carbon particles 80 75 Carbon nano-tubes   0  5 Volume specific resistance (mÙ · cm)2060 20

With the use of the above-described MEA and separator plates, a cellstack was assembled by stacking 50 cells in the same manner as inExample 1. However, the clamping force of the cell stack per area of theseparator plate was 10 kgf/cm².

The polymer electrolyte fuel cell of this example thus produced was heldat 85° C., and a hydrogen gas humidified and heated to a dew point of83° C. was supplied to the anode, while the air humidified and heated toa dew point of 78° C. was supplied to the cathode. As a result, an opencircuit voltage of 50 V was obtained during no load at which a currentis not output to the outside.

A continuous power generation test was performed under the conditionsthat the fuel gas utilization ratio was 80%, the oxygen utilizationratio was 40% and the current density was 0.5 A/cm². The change in theoutput voltage with time is shown in FIG. 7. It was confirmed that thecell of this example maintained a cell output not lower than 0.5 V inaverage voltage over 8000 hours.

EXAMPLE 4

With the use of seven kinds of compositions containing the binder andcarbon nano-tubes in different amounts, separator plates were injectionmolded under the same conditions as in Example 3. The moldability andvolume resistivity of the separator plates are shown in Table 2.

TABLE 2 No. 2-1 2-2 2-3 2-4 2-5 2-6 2-7 2-8 Composition Binder 15 25 3525 25 25 25 45 (wt %) Carbon 80 70 60 70 74.5 65 55 50 particles Carbon5 5 5 0 0.5 10 20 5 nano- tubes Carbon 0 0 0 5 0 0 0 0 fibers (60-70 μm)Volume specific — 22 70 — 30 18 18 500 resistance (mÙ · cm) MoldabilityX ⊚ ⊚ X ⊚ ◯ Δ ⊚

When the amount of the binder was less than 20 wt %, the flowabilityduring injection deteriorated extremely, and injection molding wasdifficult. When the amount of the binder exceeded 40 wt %, themoldability was improved, but the volume resistivity of the moldedseparator plate deteriorated extremely. When the amount of carbonnano-tubes was within a range of 0.5 wt % to 10 wt %, a reduction in thevolume resistivity of the resultant separator plates was significant.However, even when the amount of carbon nano-tubes was increased to 10wt % or more, the effect of reducing the volume resistivity was small.When other carbon fibers (with a fiber length of 60 to 70 μm) were mixedinstead of the carbon nano-tubes, the flowability of the compositiondeteriorated extremely, and injection molding was difficult.

Based on the results, as a preferred composition, molding pellets wereproduced by sufficiently heating and kneading 25 wt % of polyphenylenesulfide as a binder, 70 wt % of conductive carbon particles with anaverage particle diameter of 50 to 200 μm, and 5 wt % of carbonnano-tubes with a fiber diameter of 10 to 30 nm and a fiber length of 1to 10 μm. The pellets were charged into the injection molding machine,and injection molded in the predetermined die to produce the conductiveseparator plate. The injection pressure was 1600 kgf/cm², the dietemperature was 150° C., and the molding time was 20 seconds.

With the use of the separator plates thus produced, a cell stack wasassembled in the same manner as in Example 3, and the characteristicsthereof were evaluated under the same conditions as in Example 3. As aresult, it was confirmed that the cell of Example 4 had thecharacteristics as good as those of the cell of Example 3. Moreover, theseparator plates of Example 4 had superior toughness, abrasionresistance and impact resistance compared to the separator plates ofExample 3.

EXAMPLE 5

In this example, the molding pellets of Example 4 were used, but afluorine-based releasing agent and carbon nano-tubes were applied to thesurface of the die for injection molding in advance. The carbonnano-tubes transferred from the die were bonded to the surface of theseparator plate molded under the same conditions as in Example 4, andthe most part of the carbon nano-tubes protruded from the surface of thecarbon separator plate. With the use of such a separator plate, it ispossible to increase the contact point between the separator plate andthe gas diffusion layer, thereby significantly reducing the contactresistance. Moreover, by treating the surface of a separator plate thatwas produced by ordinary injection molding, at about 500° C. to removethe resin layer from the surface, the carbon nano-tubes protrude fromthe surface of the separator plate in the same manner, and the effect ofreducing the contact resistance is obtained. The separator platesaccording to Example 5 had superior toughness, abrasion resistance andimpact resistance compared to the separator plates of Example 3.

EXAMPLE 6

In this example, molding pellets comprising compositions containingvarious kinds of conductive carbon particles having differentlength/width and metallic filler were produced, and separator plateswere produced by injection molding the molding pellets. The injectionpressure was 1000 kgf/cm², the die temperature was 150° C., and themolding time was 20 seconds. The molded separator plates were immersedin a 3% hydrochloric acid aqueous solution for two hours, and thenwashed with water and dried to remove the silver powder exposed on thesurfaces. When the cross sections of the separator plates were observed,the silver powder was present between the carbon particles in theseparator plates, and the silver powder particles were not continuouslyconnected to each other. Further, it was confirmed that only the silverpowder exposed on the surfaces had been removed.

Table 3 shows the volume resistivity of the separator plates produced asdescribed above and the moldability. With the addition of the metallicfiller, even when the amount of the binder was increased, the separatorplates had similar volume resistivity, and improved moldability comparedto the separator plates containing no metallic filler. It is thus clearthat the addition of metallic filler has an advantageous effect. Withthe use of a filler comprising conductive carbon particles or silverpowder whose width exceeds 200 μm, the material has a filling defect inthe thinnest portion of the molded separator plate. Therefore, there areproblems in the moldability and the gas permeability of the resultantseparator plate. For example, in the separator plate shown in FIGS. 2and 3, the thinnest portion is a portion having the gas flow channel 29on one surface and the gas flow channel 33 on the other surface. Thethickness of the thinnest portion is 0.6 mm in the separator plate ofExample 1, and 1.0 mm in the separator plate of Example 3. It was thusfound that the diameter of the filler should preferably be set to avalue which is not greater than ⅓ of the thinnest portion of theseparator plate and not greater than about 200 μm.

TABLE 3 No. 3-1 3-2 3-3 3-4 3-5 3-6 3-7 3-8 3-9 3-10 Composition Binder 30 30 30 30 30 40 40 40 40 40 (wt %) Carbon 100/100  70 particle200/100 70 50 50 50 50 50 length/ 300/100 70 width 400/200 70 (μm)500/250 70 Silver powder  50/50 10 length/ 100/50 10 width 200/50 10(μm) 400/200 10 500/250 10 Volume specific resistance (mÙ · cm) 100 3025 25 25 50 30 30 30 30 Moldability ◯ ◯ ◯ ◯ X ⊚ ⊚ ⊚ ⊚ X

EXAMPLE 7

In this example, molding pellets comprising compositions containingvarious kinds of conductive carbon particles having different particlesize distribution and conductive carbon fine particles were prepared,and separator plates were produced by injection molding the moldingpellets under the same conditions as in Example 6. Table 4 shows themoldability and the volume resistivity of the resultant separatorplates.

When the peak of the particle size distribution of the conductive carbonparticles exceeded an average diameter of 200 μm, a molding defectoccurs in the thinnest portion of the separator plate, and consequentlythere arises a problem of gas permeability. When the peak of theparticle size distribution of the conductive carbon particles is lessthan an average diameter of 50 μm, the volume resistivity deteriorates.However, it was found that it is possible to significantly lower thevolume resistivity by mixing carbon fine particles that have the peak ofthe particle size distribution at an average diameter of 30 to 100 nmwith carbon particles having the peak of the particle size distributionat an average diameter of 50 to 200 nm. The fine particles used herewere ketjen black manufactured by LION CORPORATION.

According to the results, an example of the preferred composition ofmolding pellets comprises 40 wt % of polyphenylene sulfide as thebinder, 50 wt % of conductive carbon particles with an average diameterof 50 to 200 μm, 6 wt % of silver powder with a length of 100 to 250 μmand a width not more than 50 μm, and 4 wt % of ketchen black(manufactured by LION CORPORATION.).

EXAMPLE 8

In this example, molding pellets comprising compositions containingconductive carbon particles having a preferred particle sizedistribution were produced, and separator plates were molded under thesame conditions as in Example 6. Table 5 shows the moldability and thevolume resistivity of the resultant separator plates.

When the amount of the binder was less than 20 wt %, the flowabilityduring injection deteriorated extremely, and injection molding wasdifficult. When the amount of the binder was 45 wt % or more, themoldability improved, but the volume resistivity of the resultantseparator plates deteriorated extremely. Further, when 0.5 to 15 wt % ofmetallic filler was mixed, a significant reduction in the volumeresistivity was observed. However, even when the amount of the metallicfiller was increased to 15 wt % or more, the effect does not change. Inorder to reduce the influence of outflow of metal ions, it is preferableto set the upper limit of the silver powder to 15 wt %. When 0.5 to 10wt % of conductive carbon fine particles with an average diameter of 30to 100 μm were mixed, a significant reduction in the volume resistivitywas observed. However, even when the amount of the carbon fine particleswas increased to 10 wt % or more, the effect does not change. Since thecarbon fine particles have a small bulk density, it is difficult toevenly disperse and mix the carbon fine particles. It is thereforepreferable to set the upper limit of the carbon fine particle to 10 wt%.

According to the results, an example of the preferred composition ofmolding pellets comprises 40 wt % of polyphenylene sulfide as thebinder, 50 wt % of conductive carbon particles with an average particlediameter of 50 to 200 μm, 4 wt % of conductive carbon fine particleswith an average diameter of 30 to 100 μm, and 6 wt % of silver powder asthe metallic filler.

The molded separator plates were immersed in a 3% hydrochloric acidaqueous solution for two hours, and then washed with water and dried toremove the silver powder exposed on the surfaces thereof.

Fuel cells were assembled using the separator plates produced from thepellets having the preferred compositions of Examples 7 and 8. Thesefuel cells similar to that of Example 3 exhibited substantially the samecharacteristics as the fuel cell of Example 3, under the sameconditions.

TABLE 4 No. 4-1 4-2 4-3 4-4 4-5 4-6 Composition Binder  40 40 40 40 4040 (wt %) Carbon particle   <50 μm  60 56 size distribution 50-200 μm 6056 peak average   200 μm< 60 56 diameter 30-100 nm  4  4  4 Volumespecific resistance (mÙ · cm) 100 40 40 15 15 15 Moldability ⊚ ⊚ X ⊚ ⊚ X

TABLE 5 No. 5-1 5-2 5-3 5-4 5-5 5-6 5-7 5-8 5-9 5-10 Composition Binder15 25 40 45 35 35 35 35 35 35 (wt %) Carbon particle 50-200 μm 75 65 5045 64.5 50 45 64.5 55 50 size distribution 30-100 μm  0  0  0  0 0  0  00.5 10 15 peak average diameter Silver powder 10 10 10 10 0.5 15 30 0  0 0 Volume specific resistance (mÙ · cm) 15 25 30 500 35 27 35 20 15 15Moldability X ◯ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to produce separatorplates by injection molding a composition containing a binder composedof a thermoplastic resin, instead of a conventional technique of cuttingcarbon plates, thereby achieving a significant reduction in the cost.Moreover, it is possible to minimize the increase in the volumeresistivity. Furthermore, the separator plates have toughness, abrasionresistance and impact resistance, and contribute to an improvement inthe yield of the assembly of fuel cells.

1. A polymer electrolyte fuel cell comprising: a hydrogen ion conductivepolymer electrolyte membrane; a pair of electrodes sandwiching saidhydrogen ion conductive polymer electrolyte membrane therebetween; and apair of conductive separator plates including means for supplying anddischarging a fuel gas to and from one of said electrodes and supplyingand discharging an oxidant gas to and from the other electrode, whereinsaid conductive separator plates comprise molded plates of a compositioncomprising a binder, conductive carbon particles whose average particlediameter is not less than 50 μm and not more than ⅓ of a thickness of athinnest portion of said conductive separator plate, and carbonnanotubes; wherein said carbon nanotubes have a diameter of 10 to 30 nmand a length of 1 to 10 μm; and wherein said binder is an acid-resistingresin.
 2. The polymer electrolyte fuel cell as set forth in claim 1,further comprising carbon fine particles having a peak of particle sizedistribution at an average diameter of 30 to 100 nm.
 3. The polymerelectrolyte fuel cell as set forth in claim 1, wherein said compositionfurther comprises a metallic filler.
 4. The polymer electrolyte fuelcell as set forth in claim 3, wherein said metallic filler has alength-to-width ratio of not less than 2, and the width is not more than⅓ of the thickness of the thinnest portion of said conductive separatorplate.
 5. The polymer electrolyte fuel cell as set forth in claim 3,wherein the metallic filler exposed on surfaces of said separator plateshas been removed.
 6. The polymer electrolyte fuel cell as set forth inclaim 1, wherein said binder is a gastight elastic body having an acidresistance.
 7. The polymer electrolyte fuel cell as set forth in claim6, wherein said gastight elastic body comprises a polymer elastic bodyincluding polyisobutylene represented by the formula (1) or an ethylenepropylene random copolymer represented by the formula (2) as amain-chain skeleton:

where X and Y are polymerizable functional groups, and m is an integernot less than 1 representing repetition number of isobutylene oligomer:

where X and Y are polymerizable functional groups, and 1 and m areintegers not less than
 1. 8. The polymer electrolyte fuel cell as setforth in claim 1, wherein said composition comprises 20 to 45 wt % ofbinder, 50 to 74 wt % of carbon nanotubes, and 0.5 to 10 wt % ofconductive carbon fine particles.
 9. The polymer electrolyte fuel cellas set forth in claim 8, further comprising 0.5 to 15 wt % of metallicfiller.
 10. The polymer electrolyte fuel cell as set forth in claim 1,wherein said carbon nanotubes partially protrude from surfaces of saidseparator plates.